The London mayor is considering legal action over the billboard, which carries a smaller caption reading: âNo matter what you look like.â

The poster, on an advert for affair dating service Ashley Madison, appeared in Camden High Street in north London yesterday as part of a campaign to be rolled out across Britain.

Bill Clinton and Arnold Schwarzenegger have previously been used by the US company. Silvio Berlusconi âs face is due to front an Italian advert for the infidelity site, which has 360,000 users in Britain and declares: âLife is short. Have an affair.â

âIf you choose public life and to have affairs in public life, then you should have an expectation people will talk about it,â Noel Biderman, head of Ashley Madison, told Metro.

âIn just about every country you can think of where Ashley Madison is growing, there is a politician of some note whoâs had anÂ affair. Affairs are incredibly common.â

A spokesman for the mayorâs office confirmed legal advice is being sought.Twice-married Mr Johnson, 47, has a history of alleged infidelity.

He was sacked from the shadow cabinet in 2004 for lying over his alleged affair with writer Petronella Wyatt.

Two years later, it emerged the father of four had a fling with Times Higher Education Supplement journalist Anna Fazackerley.

Last year, he was alleged to have fathered a love child with art consultant Helen Macintyre.

The Advertising Standards Authority says it has yet to receive any complaints about the advert.